"Because the Ministry of Defence's finances are better run and better managed, and because we have found savings, we will be able to purchase an additional C-17 for the RAF.

"This aircraft is becoming an absolutely brilliant workhorse for the RAF in terms of bringing men and material into a war zone like Afghanistan, but also evacuating civilians in times of need.

"It's an important investment for the country and I'm glad to announce we can make it today."

The purchase of an extra C-17 will help to support the airbridge which transports Armed Forces personnel and equipment between the UK and Afghanistan.

The newest C-17 is currently being built by Boeing in the USA and is due to come off the production line next month. It is then expected to enter service with the RAF in July 2012.

The aircraft, which will cost £200m, will be based at RAF Brize Norton in Oxfordshire along with the other aircraft in the fleet which are operated by 99 Squadron. The C-17 aircraft can fly over 4,500 nautical miles (8,300km), meaning it can cover the distance beween the UK and Helmand province in the one flight.

Defence Secretary Philip Hammond said:

"The C-17 has shown its worth in Afghanistan ensuring that our troops are given the fastest, most efficient passage home.

"It is a vastly impressive aircraft that can carry three Apache helicopters, or take a whole Chinook helicopter.

"This latest addition to the RAF fleet will further strengthen the vital airbridge between Britain and Afghanistan, ensuring critical deliveries are made to the front line.

"Buying equipment off-the-shelf allows us to quickly deliver equipment that our troops need on operations at best value prices for the taxpayer and is a great example of the White Paper, announced last week, in action."

As well as transporting troops and equipment, C-17s can also be converted rapidly to offer intensive care provision in support of the aeromedical airbridge.

One aircraft is on standby 24-hours-a-day to fly seriously injured personnel back to the UK at short notice. The aircraft can also be deployed to help with humanitarian and disaster relief efforts around the world.